Artificial subsurface drainage systems have been in use in the Midwest for over 150 years. Subsurface drainage is common throughout the flat and gently rolling areas of the Midwest, as well as in other parts of the country. Subsurface drainage improves trafficability, enhances field conditions for timelier planting and harvesting operations, and helps decrease crop damage that can result from saturated soil and standing water. These systems facilitate crop production in areas that would be otherwise unsuitable, and increase yield in others. Almost invariably, they were designed for the sole purpose of quickly removing excess water from the plant root zone to prevent wet stress and to improve crop yields, but with no consideration of their effects on water quality.
The addition of properly designed and constructed water control structures to a subsurface drainage system allows the drainage outlet to be artificially set at any level between the ground surface and the drain depth. Raising the outlet after planting helps keep water available for plant use longer than does "free," uncontrolled subsurface drainage. This practice also can be used to recharge the water table between growing seasons. Most existing subsurface drainage systems can be retrofitted for controlled drainage. Controlled drainage systems require a moderate level of management so that excess soil-water conditions following heavy rainfall can be avoided.
Based upon research conducted throughout the Midwest, controlled drainage may provide upwards of 50% reduction in nitrate losses from subsurface drained cropland, and helps to increase corn and soybean yields.
By installing water control structures, by modifying existing drainage systems, or installing new systems, farmers can better manage hydrologic factors as well as the quantities and timing of water releases. Farmers find that these modifications reduce losses of nutrients, such as nitrates and phosphates, and conserve water in the soil for crop uptake and use during drier periods.
Management of subsurface drainage systems during the winter non-cropping months can typically be accomplished without much difficulty by merely adjusting the drainage outlet to a constant, shallow depth (e.g., 18 inches). Beginning in the spring, periodic adjustment of a deeper drainage outlet control depth may be required as new crop roots develop; this will vary with crop and soils.
Over the next two decades, producers will be replacing or installing new drainage systems to replace the older ones which are approaching 60 to 100 years old. Therefore, it is critical to increase awareness of drainage water management to ensure that the second generation of drainage technologies and practices will solve the problems created by the first generation.
